1. Field of the Invention
The present invention relates to a precursor for a Nb3Sn superconductor wire, a method for manufacturing the same, a Nb3Sn superconductor wire and a superconducting magnet system. More particularly, the invention relates to a precursor for a Nb3Sn superconductor wire to be manufactured by internal diffusion method, which is provided with both excellent critical current characteristics and magnetic stability, a method for manufacturing the same, a Nb3Sn superconductor wire and a superconducting magnet system. Herein, the “precursor” is a structure prior to final formation of the superconductor wire by the heat treatment.
2. Related Art
As technical fields to which the superconductor wire is applied, there are nuclear magnetic resonator analyzing apparatus and magnetic resonance imaging apparatus, for which the superconductor wire is used because of the magnetic field with a time stability in a persistent current mode. Particularly, in a permanent magnet for a nuclear magnetic resonance apparatus (NMR), application of the superconductor wire having high current density in high magnetic field is demanded, so as to realize higher resonance frequency, higher resolution performance and downsizing of the system.
Here, the Nb3Sn superconductor wire manufactured by the internal diffusion method has been known. According to the internal diffusion method, the Nb3Sn superconductor wire is manufactured by disposing Nb-based (i.e. Nb or Nb-alloy) cores, Sn-based (i.e. Sn or Sn-alloy) cores within a Cu-based (i.e. Cu or Cu-alloy) matrix in a precursor cross section, and reacting Sn of the Sn-based core with Nb via the Cu-based matrix by heat treatment, thereby generating Nb3Sn. The internal diffusion method is advantageous in comparison with the bronze method, since a large quantity of Sn can be provided so that a thickness of the generated Nb3Sn is large and the critical current density can be increased.
While the bronze method is a method for manufacturing Nb3Sn by reacting a precursor containing Cu—Sn-alloy and Nb-based cores, the internal diffusion method is a method for manufacturing Nb3Sn by reacting the precursor having the aforementioned structure, so that there is no bronze (Cu—Sn-alloy) in the precursor.
However, in the internal diffusion method, there is a disadvantage in that a space required for disposing Sn is greater than that in the bronze method, so that the Nb-based cores should be disposed in a limited space. Therefore, in the case that the Nb-based cores having the same cross sectional area as that in the bronze method are disposed, a spacing between the respective Nb-based cores is inevitably reduced. Due to these factors, adjacent Nb3Sn superconductor filaments are magnetically coupled to each other, so that alternative current (AC) loss is increased. Further, in the case that the degree of the magnetic coupling between the superconductor filaments is extremely large, the superconductor wire causes a magnetic jump (flux jump). As a result, when such a superconductor wire is applied to the magnet system, it is impossible to energize the superconductor wire up to the originally designed critical current.
As the Nb3Sn superconductor wire made by the internal diffusion method, by which the AC loss can be reduced while high critical current density is maintained, a Nb3Sn superconductor wire in which Sn elements and Nb elements are dispersively disposed and the Nb elements are divided into plural sections by the Sn elements in the precursor cross section has been known. For example, Japanese Patent Laid-Open No. 2010-97902 (JP-A 2010-97902) discloses such a Nb3Sn superconductor wire. JP-A 2010-97902 discloses that, in the cross section of the Nb3Sn superconductor wire obtained by heat-treating the precursor, a Nb3Sn core-dense region is divided by non-superconducting regions in which Sn has existed before reaction, so that the coupling between the respective superconducting regions is suppressed.